Explanation for bond lengths in trans-hexatriene
Hexatriene is an unsaturated hydrocarbon with six carbon atoms and five carbon-carbon bonds, three of which are double bonds.
However, the bond lengths of the $ce{C=C}$ bonds are not the same. The middle $ce{C=C}$ bond has a length of 137 pm while the $ce{C=C}$ bonds at the end of the molecule have lengths of 134 pm, the length of a standard $ce{C=C}$ bond. The two carbon-carbon single bonds are 146 pm long, also off from the standard 154 pm length of carbon-carbon single bonds.
Clayden's organic chemistry hints that the explanation has to do with the molecular orbits formed and the conjugation system in the molecule. However, I do not fully understand this explanation.
Why do these carbon-carbon bonds show this unusual bond length behavior? A thourough explanation using MO theory would be appreciated.
References
Clayden, J., Greeves, N., Warren, S. Organic chemistry, 2nd ed.; Oxford University Press: New York, 2012.
organic-chemistry bond molecular-orbital-theory
edited 3 hours ago
orthocresol♦
37.9k7111227
asked 4 hours ago
Ethiopius
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add a comment |
Hexatriene is an unsaturated hydrocarbon with six carbon atoms and five carbon-carbon bonds, three of which are double bonds.
However, the bond lengths of the $ce{C=C}$ bonds are not the same. The middle $ce{C=C}$ bond has a length of 137 pm while the $ce{C=C}$ bonds at the end of the molecule have lengths of 134 pm, the length of a standard $ce{C=C}$ bond. The two carbon-carbon single bonds are 146 pm long, also off from the standard 154 pm length of carbon-carbon single bonds.
Clayden's organic chemistry hints that the explanation has to do with the molecular orbits formed and the conjugation system in the molecule. However, I do not fully understand this explanation.
Why do these carbon-carbon bonds show this unusual bond length behavior? A thourough explanation using MO theory would be appreciated.
References
Clayden, J., Greeves, N., Warren, S. Organic chemistry, 2nd ed.; Oxford University Press: New York, 2012.
organic-chemistry bond molecular-orbital-theory
edited 3 hours ago
orthocresol♦
37.9k7111227
asked 4 hours ago
Ethiopius
7817
add a comment |
Hexatriene is an unsaturated hydrocarbon with six carbon atoms and five carbon-carbon bonds, three of which are double bonds.
However, the bond lengths of the $ce{C=C}$ bonds are not the same. The middle $ce{C=C}$ bond has a length of 137 pm while the $ce{C=C}$ bonds at the end of the molecule have lengths of 134 pm, the length of a standard $ce{C=C}$ bond. The two carbon-carbon single bonds are 146 pm long, also off from the standard 154 pm length of carbon-carbon single bonds.
Clayden's organic chemistry hints that the explanation has to do with the molecular orbits formed and the conjugation system in the molecule. However, I do not fully understand this explanation.
Why do these carbon-carbon bonds show this unusual bond length behavior? A thourough explanation using MO theory would be appreciated.
References
Clayden, J., Greeves, N., Warren, S. Organic chemistry, 2nd ed.; Oxford University Press: New York, 2012.
organic-chemistry bond molecular-orbital-theory
edited 3 hours ago
orthocresol♦
37.9k7111227
asked 4 hours ago
Ethiopius
7817
Hexatriene is an unsaturated hydrocarbon with six carbon atoms and five carbon-carbon bonds, three of which are double bonds.
However, the bond lengths of the $ce{C=C}$ bonds are not the same. The middle $ce{C=C}$ bond has a length of 137 pm while the $ce{C=C}$ bonds at the end of the molecule have lengths of 134 pm, the length of a standard $ce{C=C}$ bond. The two carbon-carbon single bonds are 146 pm long, also off from the standard 154 pm length of carbon-carbon single bonds.
Clayden's organic chemistry hints that the explanation has to do with the molecular orbits formed and the conjugation system in the molecule. However, I do not fully understand this explanation.
Why do these carbon-carbon bonds show this unusual bond length behavior? A thourough explanation using MO theory would be appreciated.
References
Clayden, J., Greeves, N., Warren, S. Organic chemistry, 2nd ed.; Oxford University Press: New York, 2012.
organic-chemistry bond molecular-orbital-theory
organic-chemistry bond molecular-orbital-theory
edited 3 hours ago
orthocresol♦
37.9k7111227
asked 4 hours ago
Ethiopius
7817
edited 3 hours ago
orthocresol♦
37.9k7111227
asked 4 hours ago
Ethiopius
7817
edited 3 hours ago
orthocresol♦
37.9k7111227
edited 3 hours ago
orthocresol♦
37.9k7111227
edited 3 hours ago
orthocresol♦
37.9k7111227
37.9k7111227
asked 4 hours ago
Ethiopius
7817
asked 4 hours ago
Ethiopius
7817
asked 4 hours ago
Ethiopius
7817
7817
add a comment |
add a comment |
1 Answer
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If you derive the π-type molecular orbitals of hexatriene, the three lower-energy MOs which are filled would look something like this (image from p 33 of Fleming's Molecular Orbitals and Organic Chemical Reactions, Reference Edition):
I suspect what Clayden is getting at is that in the second MO, there is some antibonding character between C3 and C4, whereas the C1/C2 and C5/C6 interaction is purely bonding.
answered 2 hours ago
orthocresol♦
37.9k7111227
And this antibonding interaction between C3 and C4 would cause the double bond to have slight single bond character, which would explain the longer than usual bond length for the C3=C4 bond?
– Ethiopius
2 hours ago
2
Yes, pretty much. So it is something like a 1.99-bond, if that makes any sense. (That number's made up, of course.)
– orthocresol♦
2 hours ago
add a comment |
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1 Answer
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If you derive the π-type molecular orbitals of hexatriene, the three lower-energy MOs which are filled would look something like this (image from p 33 of Fleming's Molecular Orbitals and Organic Chemical Reactions, Reference Edition):
I suspect what Clayden is getting at is that in the second MO, there is some antibonding character between C3 and C4, whereas the C1/C2 and C5/C6 interaction is purely bonding.
answered 2 hours ago
orthocresol♦
37.9k7111227
And this antibonding interaction between C3 and C4 would cause the double bond to have slight single bond character, which would explain the longer than usual bond length for the C3=C4 bond?
– Ethiopius
2 hours ago
2
Yes, pretty much. So it is something like a 1.99-bond, if that makes any sense. (That number's made up, of course.)
– orthocresol♦
2 hours ago
add a comment |
If you derive the π-type molecular orbitals of hexatriene, the three lower-energy MOs which are filled would look something like this (image from p 33 of Fleming's Molecular Orbitals and Organic Chemical Reactions, Reference Edition):
I suspect what Clayden is getting at is that in the second MO, there is some antibonding character between C3 and C4, whereas the C1/C2 and C5/C6 interaction is purely bonding.
answered 2 hours ago
orthocresol♦
37.9k7111227
And this antibonding interaction between C3 and C4 would cause the double bond to have slight single bond character, which would explain the longer than usual bond length for the C3=C4 bond?
– Ethiopius
2 hours ago
2
Yes, pretty much. So it is something like a 1.99-bond, if that makes any sense. (That number's made up, of course.)
– orthocresol♦
2 hours ago
add a comment |
If you derive the π-type molecular orbitals of hexatriene, the three lower-energy MOs which are filled would look something like this (image from p 33 of Fleming's Molecular Orbitals and Organic Chemical Reactions, Reference Edition):
I suspect what Clayden is getting at is that in the second MO, there is some antibonding character between C3 and C4, whereas the C1/C2 and C5/C6 interaction is purely bonding.
answered 2 hours ago
orthocresol♦
37.9k7111227
If you derive the π-type molecular orbitals of hexatriene, the three lower-energy MOs which are filled would look something like this (image from p 33 of Fleming's Molecular Orbitals and Organic Chemical Reactions, Reference Edition):
I suspect what Clayden is getting at is that in the second MO, there is some antibonding character between C3 and C4, whereas the C1/C2 and C5/C6 interaction is purely bonding.
answered 2 hours ago
orthocresol♦
37.9k7111227
answered 2 hours ago
orthocresol♦
37.9k7111227
answered 2 hours ago
orthocresol♦
37.9k7111227
answered 2 hours ago
orthocresol♦
37.9k7111227
37.9k7111227
And this antibonding interaction between C3 and C4 would cause the double bond to have slight single bond character, which would explain the longer than usual bond length for the C3=C4 bond?
– Ethiopius
2 hours ago
2
Yes, pretty much. So it is something like a 1.99-bond, if that makes any sense. (That number's made up, of course.)
– orthocresol♦
2 hours ago
add a comment |
And this antibonding interaction between C3 and C4 would cause the double bond to have slight single bond character, which would explain the longer than usual bond length for the C3=C4 bond?
– Ethiopius
2 hours ago
2
Yes, pretty much. So it is something like a 1.99-bond, if that makes any sense. (That number's made up, of course.)
– orthocresol♦
2 hours ago
And this antibonding interaction between C3 and C4 would cause the double bond to have slight single bond character, which would explain the longer than usual bond length for the C3=C4 bond?
– Ethiopius
2 hours ago
And this antibonding interaction between C3 and C4 would cause the double bond to have slight single bond character, which would explain the longer than usual bond length for the C3=C4 bond?
– Ethiopius
2 hours ago
2
2
Yes, pretty much. So it is something like a 1.99-bond, if that makes any sense. (That number's made up, of course.)
– orthocresol♦
2 hours ago
Yes, pretty much. So it is something like a 1.99-bond, if that makes any sense. (That number's made up, of course.)
– orthocresol♦
2 hours ago
add a comment |
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